Switching-mode power supplies, also known as switched-mode power supplies (SMPS), are widely used in applications that rely on DC-DC electrical power conversion. A typical SMPS includes a controllable power switch, such as a MOSFET (metal-oxide semiconductor field-effect transistor), a rectifier, output circuitry, and a controller. The controller is used to switch the power switch between its ON state, in which it is conducting current, and its OFF state, in which it is not conducting current. The alternating current is rectified and filtered, via the rectifier and output circuitry, and the filtered DC is supplied to a load.
In some SMPS the rectifier is implemented using a synchronous rectifier. That is, another controllable switch, such as a MOSFET, that is also controlled, via the controller, to switch between its ON and OFF states. Using a synchronous rectifier is advantageous, in that it exhibits lower conduction losses and fairly linear resistance characteristics. One drawback of the synchronous rectifier topology is that excessive power dissipation can occur in the synchronous rectifier under certain conditions, such as an output short-circuit, power down, and back bias conditions.
Hence, there is a need for a switching power supply that is implemented with a synchronous rectifier that does not dissipate undesirably high power under certain conditions, such as an output short-circuit, power down, and back bias conditions. The present invention addresses at least this need.